There are broadly three different types of liquid crystalline material: nematic, cholesteric, and smectic. The types are distinguished by differences in molecular ordering. Such materials only show a liquid crystal phase over a limited temperature range between the solid and isotropic liquid phases. Within the liquid crystal phase temperature range, a material may exhibit one or more of the nematic, cholesteric or smectic phase types. Normally, a material is chosen such that it forms only one type of liquid crystal phase over its working temperature range.
Liquid crystalline elastomers combine the various broken symmetries of liquid crystalline phases with the elasticity of polymer networks. One obvious effect is that the single crystal elastomers can undergo spontaneous shape changes when they undergo a liquid crystalline transition. There are many more subtle effects in the interplay between the fluctuations of the familiar liquid crystalline and elastic degrees of freedom (for example, and perfect single crystal nematic elastomer can, in theory, exhibit elastic moduli of zero when deformed in certain directions. These dramatic effects are, of course, drastically influenced by the disorder, which makes them perfect for studying quenched disorder.
Biopolymer networks are found all over nature. For example, the cytoskeleton is supported by a network of actin, which is a semi-flexible polymer with globular proteins as a monomer unit. These networks are proving to be ideal model systems for understanding the physics of semi-flexible polymers, both in solution and cross-linked network states.
The fibrous proteins can be considered to be a special class of proteins that serve important structural functions in the extracellular environment. In living organisms some of these proteins, such as collagens, are found in thin layers, sandwiched between other extracellular biomaterials. When studying the physical chemistry of extracellular fibrous proteins in vitro the use of a two-dimensional thin film or interfacial environment will help the proteins self-assemble more efficiently by providing a restricted environment in comparison to three-dimensional bulk systems. It is thus beneficial to study the synergistic interaction between the behavior of fibrous proteins in dimensionally restricted environments (such as thin films or two-dimensional layers) and the generation of structure and long range order through self-assembly.
Different conformations can be stabilized by an interface, such as an extended chain β-sheet conformation, which maximizes the protein's spreading and surface area. If the protein or model polypeptide has hydrophobic side chains, and can readily take on a stable a-helical conformation, α-helices will be stable at the interface. Biridi, K. S. Journal of Colloid and Interface Science 1973, 43, 545; Cheesman, D. F.; Davies, J. T. Advan. Protein Chem. 1954, 9, 439; Jacuemain, D.; Wolf, S. G.; Leveiller, F.; Lahav, M.; Leiserowitz, L.; Deutsch, M.; Kjaer, K.; Als-Nielsen, J. Journal of the American Chemical Society 1990, 112, 7724-7736; Loeb, G. I. Journal of Colloid and Interface Science 1968, 26, 236; Loeb, G. I. Journal of Colloid and Interface Science 1969, 31, 572; Macritchie, F. Adv. Coll. Int. Sci. 1986, 25, 341-382; Magdassi, S.; Garti, N. Surface Activity of Proteins; Magdassi, S.; Garti, N., Ed.; Marcel Dekker: New York, 1991; Vol. 39, pp 289 -300; Malcolm, B. R. Nature 1962, 4195, 901; Malcolm, B. R. Soc. Chem. Ind. London 1965, 19, 102; Malcolm, B. R. Progress in Surface and Membrane Science 1971, 4, 299; Murray, B. S. Coll. Surf. A 1997, 125,73-83; Murray, B. S.; Nelson, P. V. Langmuir 1996, 12, 5973-5976; Weissbuch, I.; Berkovic, G.; Leiserowitz, L.; Lahav, M. Journal of the American Chemical Society 1990, 112, 5874-5875; Wustneck, R.; Kragel, J.; Miller, R.; Wilde, P. J.; Clark, D. C. Coll. Surf. A 1996, 114, 255-265. The influence of side chain character in stabilizing an interfacial conformation suggests that hydropathicity can be used as a determinant for interfacial conformation. Carrying this idea further, if a sequence of residues results in particular conformations that could exhibit surfactant behavior, these conformations should be stabilized at an interface.
Silks, and their analogues, have recently been the focus of interest for applications in biomaterials because of the intriguing properties of the silk fiber. The simplicity of their sequences lends them to be used as model fibrous proteins. Most of the studies on the properties of silks, as well as other fibrous proteins either examine gross materials properties such as mechanical properties, thermal stability and surface roughness or examine very localized chemical details in the molecule. Literature on long-range ordered “helicoids” is less abundant.
Previously we have disclosed that with B. mori silk fibroin, a threefold helical polyglycine II or polyproline II type of conformation was stabilized by the interface, even though it is not observed in bulk. Valluzzi, R.; Gido, S. P. Biopolymers 1997, 42, 705-717; Valluzzi, R.; Gido, S.; Zhang, W.; Muller, W.; Kaplan, D. Macromolecules 1996, 29, 8606-8614; Zhang, W.; Gido, S. P.; Muller, W. S.; Fossey, S. A.; Kaplan, D. L. Electron Microscopy Society of America, Proceedings 1993, 1216. The B. mori fibroin crystallizable sequence is approximately (Gly-Ala-Gly-Ala-Gly-Ser)x, and a left-handed threefold helical conformation, which is sterically reasonable, separates hydrophobic alanine and hydrophilic serine residues to opposite sides of the interface. Valluzzi, R.; Gido, S. P. Biopolymers 1997, 42, 705-717; Valluzzi, R.; Gido, S.; Zhang, W.; Muller, W.; Kaplan, D. Macromolecules 1996, 29, 8606-8614; Zhang, W.; Gido, S. P.; Muller, W. S.; Fossey, S. A.; Kaplan, D. L. Electron Microscopy Society of America, Proceedings 1993, 1216.
As a consequence of the difficulties entailed in attempting detailed surface measurements at a liquid-liquid interface, there have been few studies on the behavior of proteins at these interfaces to date. Murray and Nelson, working with a novel oil-water trough design, have published results on the comparative behavior of β-lactoglobulin and bovine serum albumin (both globular) protein films at air-water and oil-water interfaces that appear consistent with structural results obtained for fibrous proteins at air-water and oil-water interfaces. Murray, B. S. Coll. Surf. A 1997, 125, 73-83; Murray, B. S.; Nelson, P. V. Langmuir 1996, 12, 5973-5976. They found that films at the oil-water interface were more expanded and also more expansible and compressible than corresponding films at the air-water interface. This was believed to be due to a reduction in aggregation. The increased solubility of the hydrophobic groups in oil as opposed to air is cited as a reason for the greater stability of films at the oil-water interface. Shchipunov has studied phospholipids at an oil water interface, and observed that the presence of the amphiphiles results in more oil on the water side of the interface and more water on the oil side. Shchipunov, Y. A. Liquid/Liquid Interfaces and Self-Organized Assemblies of Lecithin; Shchipunov, Y. A., Ed.; CRC Press: Boca Raton, Fla., 1996, pp 295-315. The amphiphile compatibilizes the two liquids forming the interface, and in the process, the interface thickens. Both the compatibilization effect observed for the phospholipids and the stability observed for the protein films suggest that there is oil and water closely interacting with the side chains of the protein. Side chain—side chain interactions would thus be expected to be screened. Jacuemain, D.; Wolf, S. G.; Leveiller, F.; Lahav, M.; Leiserowitz, L.; Deutsch, M.; Kjaer, K.; Als-Nielsen, J. Journal of the American Chemical Society 1990, 112, 7724-7736; Malcolm, B. R. Nature 1962, 4195, 901; Murray, B. S. Coll. Surf. A 1997, 125, 73-83; Murray, B. S.; Nelson, P. V. Langmuir 1996, 12, 5973-5976; Wustneck, R.; Kragel, J.; Miller, R.; Wilde, P. J.; Clark, D. C. Coll. Surf. A 1996, 114, 255-265; Shchipunov, Y. A. Liquid/Liquid Interfaces and Self-Organized Assemblies of Lecithin; Shchipunov, Y. A., Ed.; CRC Press: Boca Raton, Fla., 1996, pp 295-315; Miller, I. R. Progress in Surface and Membrane Science 1971, 4, 299.
An aqueous-hexane interface was chosen as an initial probe of fibroin liquid-liquid interface behavior. This interface, in the absence of fibroin, is believed to be about 10 Å thick. Carpenter, I. L.; Hehre, W. J. Journal of Physical Chemistry 1990, 94, 531-536; Michael, D.; Benjamin, I. Journal of Physical Chemistry 1995, 99, 1530-1536. The silk at the aqueous-hexane interface forms a film as it ages, and this film can be picked up onto sample grids for observation in a transmission electron microscope (TEM). The hexane was expected to be a better solvent for the alanine residues in silk than the water, forcing them to the hexane side of the interface. The aqueous phase should be a better solvent for serine.